It is uncertain if anti-retrovirals fully restore immune competence in HIV+ patients. Immunotherapy has been proposed as a way to achieve this goal. IL-2 and IL-12 are cytokines being used or evaluated for this purpose. However, the ability of these cytokines to restore cell mediated immunity (CMI) in HIV+ patients remains unsettled. Thus, there is a need to explore additional approaches to further improve immune reconstitution in HIV+ patients. CD40-CD40 ligand (CD40L) is crucial for T cell-antigen presenting cell (APC) interaction. It promotes IL-12/IFN-gamma secretion, induces APC activation and regulates CD8+ T cell differentiation. Absence of functional CD40L causes immunodeficiency in humans characterized by increased susceptibility to infections with opportunistic pathogens. CD40L is relevant to HIV infection. CD40L induction is defective on CD4+ T cells from HIV+ patients. Exogenous CD40L has profound effects on CMI against HIV and opportunistic pathogens in HIV+ patients. CD40L protects CD4+ T cells from HIV infection and can inhibit HIV replication in dendritic cells. CD40L inhibits growth of opportunistic pathogens in macrophages. Thus, enhancement of CD40L signaling is an attractive candidate for immunotherapy in HIV+ patients. In Specific Aim 1, we will identify the mechanisms by which CD40L impairs replication of opportunistic pathogens in human macrophages. We will determine if CD40L also stimulates anti-microbial activity of macrophages from HIV+ patients. We will investigate if T cells from HIV+ patients are defective in their capacity to stimulate macrophage anti-microbial activity and if this activity can be enhanced by CD40L. Finally, we will examine if CD40L enhances the capacity of CD8+ T cells to inhibit HIV replication. In Specific Aim 2, we will ascertain if in vivo administration of exogenous CD40L to T cell-deficient mice increases resistance to an opportunistic pathogen. The mechanisms that mediate this effect will be elucidated. This work will provide crucial information for the evaluation of modulation of CD40L signaling as an approach to immunotherapy in HIV+ individuals.